Masks or reticles are used in the field of semiconductor fabrication in order to form a pattern of structure elements in a photosensitive resist by means of lithographic projection onto a semiconductor wafer coated with the photosensitive resist. The choice of the lateral extent of the structure elements to be formed on the semiconductor wafer is restricted due to a predetermined lower resolution limit predetermined for a particular projection system. The resolution limit depends on factors such as the exposure wavelength, the aperture size of the lens system, or, for example, on the type of illumination source of the projection system.
However, the resolution limit of a projection system can also be reduced by the use of modern lithographic techniques in the case of the masks used for exposure. Besides optical proximity correction (OPC) and/or the use of sublithographic structure elements in the vicinity of structure elements to be imaged, this primarily relates to the field of phase masks, which are also called phase shift masks.
One particular type is the so-called attenuated phase shift mask. The structure elements formed on conventional chromium masks as opaque layers on a transparent carrier substrate are embodied in semitransparent and phase-shifting fashion in the case of the attenuated masks. The term “semitransparent” is somewhat misleading since the degree of transmission, i.e., that proportion of the incident light which can penetrate through the semitransparent layer and reach the transparent carrier substrate, usually amounts to between only 3% and 8% of the intensity of the incident light beam. This intensity does not suffice to expose through the photosensitive resist at the corresponding position on the semiconductor wafer. On the other hand, the light beam that is subjected to a phase deviation of typically 180° and is attenuated on account of the low degree of transmission brings about an increased contrast enhancement compared with conventional chromium structure elements at the edges of transparent and semitransparent partial regions. In the case of attenuated phase shift masks, therefore, semitransparent structure elements can be imaged with high dimensional accuracy on a semiconductor wafer.
However, one particular problem, in the case of attenuated phase shift masks, is that the contrast-enhancing effect for dense regions of structure elements differs from that of isolated structure elements. In general, it is therefore attempted to simulate, with the aid of sublithographic structures, the presence of high densities in the vicinity of isolated structure elements. However, this gives rise to the problem of a disadvantageous increase in the expenditure on costs for the design and for the production of the mask on account of the very small writing grids for the laser or electron beam.